When a metastable atom impinges on the surface of a substance, an electron is ejected by obtaining energy released when the atom transits from an excited state to a ground state. By analyzing the energy of the electron, the surface electronic state of the substance can be analyzed. Further, information obtained by the electron which is ejected by the metastable atom beam indicates an electronic state on the outermost layer of the surface of the substance. Accordingly, the metastable atom beam can be used as a potential measuring means or the like. Further, the electron which is ejected from the surface of the substance by irradiating ultraviolet radiation is utilized for, for example, ultraviolet photoelectron spectroscopy since the average information of a substance up to a certain depth from the surface of a substance is obtained.
The inventors of the present invention have already obtained a pulse beam by developing a source of an He metastable atom beam of an electron impact type, and have also confirmed that a continuous beam with high intensity is obtained by a source of an He metastable atom beam of a discharge type (refer to "Proceedings of 44-th Applied Physics Plenary Session (1997) 426"). When an He metastable atom beam is formed by electron impact or discharge, both continuous the He metastable atom beam and ultraviolet radiation are simultaneously formed. Therefore, in order to measure the state of the surface of a substance by using the continuous beam, it is necessary to make the beam into pulses by a mechanical chopper and to discriminate the metastable atom beam from ultraviolet radiation by combining the time-of-flight method (TOF).
For example, He gas is jetted from a nozzle of about 0.1 mm in a supersonic condition. Immediately thereafter, all of the gas except a central portion thereof having high intensity is removed by a structure in a funnel-like shape having an opening portion of about 1 mm at its front end which is referred to as skimmer, and voltage of about 300 V is applied between the nozzle and the skimmer. Then a continuous metastable atom beam and ultraviolet radiation can simultaneously be formed by continuous discharge with a discharge current of about 10 mA. However, in order to obtain a pulsed metastable atom beam or pulsed ultraviolet radiation, it is necessary to integrate a mechanical chopper.
According to the structure for generating a pulse beam by using such a mechanical chopper, although the structure of the source of the He metastable atom beam per se is simple, the structure becomes complicated and large as a whole by adding the mechanical chopper. Furthermore, a distance from the source of the He metastable atom beam to the surface of a substance becomes greater, causing the intensity of the He metastable atom beam on the surface of a substance to be weakened.
Thereby, the advantage of a source of an He metastable atom beam with a simple structure is lost. Further, when the intensity of the He metastable atom beam is increased by increasing the discharge current, the discharge current has an upper limit since there is a limit to the thermal strength of the source of the He metastable atom beam. Although it is preferable when the source of the He metastable atom beam can be driven in pulses, according to the discharge characteristics, discharge start voltage is considerably higher than the maintaining voltage. Accordingly, pulse drive becomes difficult In order to measure the quantum effect with high accuracy by a metastable atom beam probe, an atom beam with high intensity is indispensable. In order to finely measure an effect caused only by a metastable atom beam, the generation of a pulsed atom beam is necessary. However, it is the actual situation that a pulsed metastable atom beam or a pulsed ultraviolet radiation which are sufficiently satisfiable have not yet been provided.